The present invention pertains to aramid fibrils, to a method of preparing said fibrils, and to paper made thereof.
Pulp is defined as fiber stem which is highly fibrillated. The fibrillated part is mentioned fibrils, which are highly entangled and have a high aspect ratio (>100) and a large surface area (8-10 m2/g) which is about 40 times that of standard filament. Thus aramid pulps are fibrillated particles that are used for making paper, gaskets, breaking lines, and the like. Pulp generally can be made from spun fiber, by performing cutting and fibrillation steps thereon. It is however advantageous to directly make pulp, without first spinning the polymer to a fiber. Such direct pulp making method has been disclosed in the art, for instance in U.S. Pat. No. 5,028,372. According to this method, an aramid pulp was made by forming a para-aramid polymer solution, extruding said solution, having an inherent viscosity between 1 and 4, onto a conveyor, incubating the solution on the conveyor until it forms a gel, and cutting this gel and isolating the pulp thereof. The polymer has a concentration of 6 to 13 wt. % of the solution and the thus obtained pulp has a specific surface area greater than 2 m2/g.
It can be envisaged that for particular applications a highly fibrillated pulp is advantageous. It would even be more advantageous that the polymeric material is fully (or essentially fully) in the fibril form, i.e., does not contain substantial amounts of fiber-like material. In other words, there is a need for “pulp” which predominantly contains the fibrillated part and no longer the fiber stems. Such material is unknown up to now. Very useful properties could be expected from such materials, such as high flexibility, high binding capacity in paper, and good porosity of papers made thereof. Further, it can be expected that such material has a considerable hardness after drying, and therefore suitable for using in composites. This material for the purpose of this invention is defined as “fibrils”.
It is well known in the art that in pulp the higher the specific surface area (SSA), the lower the Canadian Standard Freeness (CSF). Thus in the standard reference work of Yang, 1993, Wiley & Sons, ISBN 0 471 93765 7, p. 156 it is explained that the CSF decreases when the SSA increases. It is an object of the present invention to provide materials having many of the properties of pulp, but having low SSA and at the same time low CSF. It can be envisaged that such material could have unique properties for many applications, including papermaking. Such materials are unknown in the art.
Fibers with a low fibrillation degree, having low SSA are known in the art. In EP 381206, subdenier pulp-like fibers have been disclosed. These fibers have been made by standard methods using high dope concentrations and using sulfuric acid as solvent. These fibers have low SSA, but high CSF (i.e., values above 600 ml).
In EP 348996 and U.S. Pat. No. 5,028,372, pulp has been made by a method wherein the polymerization is partly performed after extrusion and orientation of the dope. The pulp has low SSA (for instance, 5.2 and 7.1 m2/g) and therefore according to Yang, p. 156, high CSF, i.e., >450 ml.